The "A" domain, or "I" domain as it is commonly called in integrins, is a approximately 200 residue protein recognition module that is present in many proteins involved in cell-cell and cell-matrix adhesion. In most cases tested, key ligand binding properties of the parent molecule are recapitulated by recombinant A domains, demonstrating that this domain is a critical element in the adhesion function. This in turn suggests that the A domain is an attractive target for therapeutic agents that would disrupt aberrant adhesion. We have previously determined the crystal structures of four members of this family and proposed a general model for ligand recognition involving the upper surface of the domain. In the integrins, a metal ion is located at the putative ligand binding interface, which we have called the "metal ion-dependent adhesion site" or MIDAS motif. In addition, we have proposed that the adhesiveness of A domains is dependent on tertiary structure changes within the A domain ("shape-shifting") that create a high affinity ligand binding surface, in which the domain switches from a "closed" to an "open conformation. We now wish to test and extend these hypotheses by determining crystal structures of A domains in complex with their ligands. Specifically, we will target complexes of the integrin alpha1 and alpha2 I domains and the von Willebrand Factor A3 domain with triple helical collagen-like peptides; the integrin alphaM I domain with fragments of fibrinogen and ICAM-1; the integrin alpha2 I domain with a fragment of laminin; and the von Willebrand Factor A1 domain with a fragment of glycoprotein lb and the snake toxin botrocetin. Successful structure determination of a range of these targets will enable us to address the following questions: What are the common and distinct features of ligand recognition by the A/I domain family? Is a metal bridge a general feature of integrin-ligand contacts? What replaces the metal in the vWF A domains? Is tertiary "shape- shifting" a common feature of A/I domains, and does this represent a mechanism for regulation? And finally, do our structural data suggest strategies for the design of small molecules that would mimic ligand binding and disrupt adhesion?